wgpu: add vec3 padding in quad.vert; fixes #511

[aentity]

This adds a vec4 in place of single float in quad uniforms to avoid metal validation errors.

[aentity]

@hecrj forgot to make PR, can review please?

[aentity]

@hecrj ping

[hecrj]

Hey, thanks!

What is exactly the cause of this issue? Could it be a bug in wgpu?

[aentity]

Wgpu should catch yes, but issue is (i think) the uniform is not correctly aligned / padded according to metal layout requirements. It could perhaps be shaderc bug, which doesn't add the necessary padding at end of the uniform, I'm not sure?

[hecrj]

I see. I believe that, before merging, we need to identify the root cause of the issue and whether it is up to us to fix it.

The proposed fix here affects all of our wgpu backends, not only Metal.

[hecrj]

Is this issue still present after #725? If it is, does wgpu provide any more hints about what is going on?

I am closing this, but feel free to reopen it if you find out the actual cause.